The present invention relates to a process for preparing an organic carboxylic acid comprising conducting carbonylation reaction of alcohol using a rhodium catalyst and an iodide co-catalyst and separating an organic carboxylic acid from the reaction product. More particularly, the invention relates to a process for preparing an organic carboxylic acid having an extremely low content of iodide.
For preparing an organic carboxylic acid such as an acetic acid, there is conventionally known a process of reacting methanol with carbon monoxide in a reaction solvent in the presence of a rhodium carbonyl complex and methyl iodide. In this process, the reaction system needs a relatively high water content (about 15%). Therefore, a high energy is required for recovering the acetic acid and, what is more important, there is involved a drawback that an expensive material of high anticorrosion must be used for the apparatus because the iodide (specifically, hydrogen iodide (HI)) produced owing to the presence of a large amount of water is a highly material-corrosive substance. On that account, as an improved process of conducting the reaction in the presence of a small amount of water, a process comprising using an alkali metal salt in combination is disclosed in Japanese Patent Laid-Open Publication No. 298549/1987 (U.S. patent application Ser. No. 874734/1986), or a process comprising using a catalyst obtained by supporting a rhodium carbonyl complex on a vinylpyridine resin having a porous crosslinked structure is disclosed in Japanese Patent Laid-Open Publication No. 306253/1993 (U.S. Pat. No. 5,334,755).
In such processes for preparing an acetic acid as mentioned above, most of iodides such as methyl iodide used as the co-catalyst and hydrogen iodide produced as a by-product through hydrolysis of the methyl iodide are removed from the acetic acid by distillation, but the iodides still remaining must be necessarily removed. For example, according to the patent of Celanese Corp. (Japanese Patent Laid-Open Publication No. 246935/1993, i.e., U.S. patent application Ser. No. 799455/1991), the industrial standard with regard to the iodide content in the acetic acid is not more than 10 ppb. Particularly when the acetic acid is used as a material of vinyl acetate, the iodide content in the acetic acid is required to be not more than 1 ppb. The reason is that a gold or palladium catalyst used in the synthesis of vinyl acetate is prevented from deactivation caused by the iodide.
For removing iodides from the acetic acid obtained as above, a method of contacting the acetic acid with an alkali metal hydroxide or the like to remove iodides contained in the acetic acid is described in Japanese Patent Laid-Open Publication No. 55695/1982 (U.S. patent application Ser. No. 200553/1971), and a method of contacting the acetic acid with methanol to remove iodides contained in the acetic acid is described in Japanese Patent Laid-Open Publication No. 23016/1977 (U.S. patent application Ser. No. 603825/1975). Further, a method for removing iodides by contacting the acetic acid containing the iodides with a macro porus strong acid cation exchange resin in which silver or mercury is immobilized to a part of the active site is disclosed in Japanese Patent Publication No. 21031/1993 (U.S. patent application Ser. No. 708992/1985), or a method for removing iodides by contacting the acetic acid containing the iodides with a polymer resin to which a metallic salt capable of being precipitated by the reaction with an iodide is bonded in the form of coordination complex is disclosed in Japanese Patent Laid-Open Publication No. 246935/1993 (U.S. patent application Ser. No. 799455/1991).
In the above method of contacting the acetic acid with an alkali metal hydroxide or methanol to remove the iodides in the acetic acid, however, there is involved a problem on the removal of alkyl iodides, though the hydrogen iodide can be removed. That is, the effect of removing alkyl iodides, e.g., methyl iodide used as the co-catalyst, is small and the whole iodine content in the acetic acid is only reduced to several tens ppb at the lowest, so that the aforesaid standard of not more than 1 ppb cannot be attained.
On the other hand, the method of using a specific ion exchange resin to which silver or mercury is bonded described in Japanese Patent Publication No. 21031/1993 and the method of using a polymer resin having a metallic salt described in Japanese Patent Laid-Open Publication No. 246935/1993 are both intended to remove hexyl iodide mainly. That is, the methods for removing iodides disclosed in Japanese Patent Publication No. 21031/1993 and Japanese Patent Laid-Open Publication No. 246935/1993 are intended to remove hardly-separated hexyl iodide having been produced as a by-product in the acetic acid of a low water content which is obtained by adding an alkali metal salt such as LiI or NaI to the reaction system. By these methods, in addition to the removal of hexyl iodide, the total content of other alkyl iodide than hexyl iodide such as methyl iodide and hydrogen iodide can be reduced to not more than 1 ppb. However, the specific resins used in these methods are high in the production cost. Moreover, if the removing methods are applied to the preparation of an acetic acid, a problem of increase of cost for preparing an acetic acid takes place, because silver or mercury must be recovered. Accordingly, a process for easily and economically preparing an organic carboxylic acid such as an acetic acid having an iodide content of not more than 1 ppb has been eagerly desired.